Some high speed cable standards such as the High-Definition Multimedia Interface HDMI specification (High-Definition Multimedia Interface Specification Version 1.3, published by Hitachi, Ltd., Matsushita Electric Industrial Co., Ltd., Philips Consumer Electronics, International B.V., Silicon Image, Inc., Sony Corporation, Thomson Inc., and Toshiba Corporation, Jun. 22, 2006) have specific limits on the resistance of power and ground lines in the cable. For example, in HDMI cables a limit of 1.8 ohms is specified for the combined resistance of the Ground line and the Power line that provides 5V power and through which power may be provided to embedded circuitry in the cable. Another example of a similar resistance limit is contained in the Universal Serial Bus (USB) 3.0 specification (Universal Serial Bus 3.0 Specification, published by Hewlett-Packard Company, Intel Corporation, Microsoft Corporation, NEC Corporation, ST-NXP Wireless, and Texas Instruments, Revision 1.0, Nov. 12, 2008) according to which the combined resistance of the Power line and the Ground line is limited to 0.4 ohms.
To achieve the specified resistance limits, the conventional approach is to decrease the gauge of the wire, i.e. increase the wire thickness, in accord with increasing cable length.
A problem with the conventional approach of decreasing the gauge of the power and ground wires is that the resulting increase in the wire thicknesses has a direct impact on the cable outer diameter and the flexibility of the cable. This size increase can be significant when active equalization of the data lines is used, which allows higher loss and relatively high gauge (low diameter) wire to be used for the signal lines.
FIG. 1a shows a schematic diagram of a shielded high speed cable 100 of the prior art, including a raw cable 102, first and second terminating ends 104.1 and 104.2 at respective first and second ends of the raw cable 102. The raw cable 102 includes wires (conductors), which extend into the first and second terminating ends 104.1 and 104.2, namely a shield 106, a power wire 108, a group of signal wires 110, and a ground wire 112. The shield 106 is an conductive layer, implemented in a form of foil or braid, for example the cable 100 cable can be wrapped in a conductive foil, most often aluminum, or it can be wrapped in a braided mesh of tiny wires. Foil and braid have different characteristics, which accounts for the fact that many cables have both braid and foil as the shield 106.
The raw cable 102 is typically surrounded by an insulating layer (not shown in FIG. 1a) made of polyvinyl chloride (PVC) or similar material.
FIG. 1b illustrates the raw cable 102 in a schematic cross-sectional view, in which the shield 106 surrounds the power line 108, the group of signal lines 110, and the ground wire 112. The group of signal lines 110 is shown to comprise 6 individual signal wires for illustrative purposes only. The actual number of signal wires varies according to the type of cable (HDMI or USB 3.0 for example). In addition to the signal wires there may also be so-called drain wires (not shown) included, which may be used for impedance control of the signal wires.
The shield 106 of the shielded high speed cable 100 is normally floating in the cable, and may be connected to a metal structure of the equipment to which the cable is connected.
Therefore there is a need in the industry for developing an improved high speed cable, which would avoid or mitigate the shortcomings of the prior art.